The invention relates to packet radio systems in general and particularly to a method and arrangement for providing independence between uplink and downlink in a packet radio network, preferably in a mobile station packet radio network such as a General Packet Radio Service (GPRS).
The GPRS is a new service in the GSM system and an object of standardization in the GSM phase 2+ in the ETSI (European Telecommunication Standard Institute). The operational environment in the GPRS comprises one or more subnetwork service areas interconnected by a GPRS Backbone Network. A subnetwork comprises a plurality of packet data service nodes, which are here referred to as GPRS support nodes (or agents), each of which is connected to the GSM mobile network so as to be able to offer a packet data service to mobile data terminal equipment via a plurality of base stations, i.e. cells. An intermediate mobile network offers circuit-switched or packet-switched data transmission between a support node and mobile data terminal equipment. The separate subnetworks are, in turn, connected to an external data network, e.g. to a PSPDN (Public Switched Packet Data Network). The GPRS service thus allows packet data transmission to be provided between mobile data terminal equipment and external data networks, with the GSM network functioning as an interface network.
With reference to FIG. 1, a typical arrangement of a GPRS network will be described. It is to be understood that the architecture in the GPRS systems is not as mature as in the GSM systems. All GPRS terminology should therefore be understood to be descriptive and not restrictive. A typical mobile station functioning as a mobile data terminal comprises a mobile station MS of a mobile network and a portable personal computer PC connected to said MS through a data interface. The mobile station can be for instance a Nokia 2110 manufactured by Nokia Mobile Phones/Finland. By means of a PCMCIA-type Nokia Cellular Datacard, manufactured by Nokia Mobile Phones/Finland, the mobile station can be connected to any portable personal computer PC provided with a PCMCIA card slot. The PCMCIA card thus provides the PC with an interface point supporting a telecommunication application protocol, such as a CCITT X.25 or an Internet Protocol IP, used on the PC. The mobile station can optionally provide a direct interface point supporting a protocol used by the PC application. A further possibility is that the mobile station MS and the computer PC are integrated into a single entity, within which the application software is provided with an interface point supporting the protocol it uses. An example of such a mobile station comprising an integrated computer is Nokia Communicator 9000, which is also manufactured by Nokia Mobile Phones/Finland.
Network elements Base Station Controller (BSC) and Mobile Switching Center (MSC) are known in a typical GSM network. The arrangement shown in FIG. 1 comprises a separate Serving GPRS Support Node (SGSN) of the GPRS service. The support node performs certain packet radio service operations in the network. Such operations include registration of the mobile stations MS in and out of the system, updating of routing areas of the mobile stations MS and routing of data packets to their correct destinations. Within the present application the term xe2x80x98dataxe2x80x99 is to be understood in a broad sense to refer to any information transmitted in a digital telecommunication system. Such information can comprise speech, inter-computer data traffic, telefax data and short program code sequencies, etc., encoded into a digital form. The SGSN node can be located at a base station BTS, a base station controller BSC or a mobile switching center MSC, or separate from all these elements. The interface between an SGSN node and a base station controller BSC is called a Gb interface. An area controlled by one base station controller BSC is called a Base Station Subsystem (BSS). Uplink refers to a direction from a mobile station MS to a network and downlink refers to a reverse direction.
In the present application, the term xe2x80x98a standard proposalxe2x80x99 refers jointly to the proposals for ETSI GPRS standards, particularly to 3.64 and to supplementing proposals sent for it, particularly to Tdoc SMG2 GPRS 174/97. One of the principles of the GPRS system is that uplink and downlink capacity deployment should be independent of each other. With reference to FIG. 2, an area, significant to the understanding of the invention, of a radio resources arrangement according to the standard proposal will be described. Traffic over an air interface Um is relayed in blocks produced by a physical layer of the OSI model. Each physical block has a duration of four GSM bursts (456 bits in total), which are sent in succession on one physical channel. The amount of data carried by a physical block depends on the channel coding to be used, for which four different coding methods have been defined, i.e. CS-1, . . . , CS-4. The different coding methods are not, however, significant to the understanding of the invention.
With reference to FIG. 3, an allocation of radio resources will be described in relation to a mobile terminating connection. Item 6.6.4.5. of the standard proposal is considered to represent the prior art. Message fields, such as a TFI and a USF, will be described later in connection with FIG. 4. In FIG. 3, time proceeds from top to bottom. The Figure shows on the right-hand side, next to messages, said logical channels on which the messages can be sent. The channel used is not, however, significant to the understanding of the invention.
In step 3-0, the network searches for a mobile station on a paging channel, i.e. it sends a Packet Paging Request message on a paging channel PPCH or PCH. In step 3-1, the mobile station sends a channel allocation request Packet Channel Request on a random access channel PRACH or RACK In the channel allocation request the mobile station can request the network to allocate to it one or two time slots. In step 3-2 the network can grant an access on a Packet Access Grant channel (immediate Assignment). In step 3-3 the mobile station sends an Logic Link Control (LLC) frame which is relayed to the SGSN to inform the SGSN that the mobile station has moved to a xe2x80x98Readyxe2x80x99 state.
The allocation of radio resources (for instance in step 3-2) involves e.g. that the network allocates to the mobile station the identifiers Temporary Flow-Identifier (TFI) and Uplink State Flag (USF). The mobile station can use one or two time slots allocated to it for sending data, or it can use the radio resources allocated to it for requesting more resources. Let us assume that the resources allocated to the mobile station by the Immediate Assignment message are not sufficient and in step 34 the network sends a Packet Resource Assignment message.
The Packet Resource Assignment message comprises e.g. the TFI identifier allocated to the connection; a list (in a bit chart form) of Packet Data Channels PDCH that the network uses for sending a packet; and a Packet Associated Control Channel PACCH that the mobile station is to use for acknowledging received RLC blocks (the channel being indicated by the USF allocated to the mobile station). In addition, the message can comprise e.g. information related to a timing advance and power control.
After the radio resources are allocated, the network sends in step 3-5 to the mobile station data packets provided with a header that comprises a polling field, i.e. a Supplementary/Polling bit S/P. If the S/P field is in a polling state xe2x80x98Pxe2x80x99 (i.e. the S/P bit is 1), the mobile station is to acknowledge in step 3-6 the received data packets with a positive or a negative acknowledgement Packet Ack/Nack. The indication xe2x80x98N timesxe2x80x99 in FIG. 3 means that the steps 3-5 and 3-6 below the dashed line are repeated for as along as the network has packets to send.
With reference to FIG. 4, the radio blocks and the fields used in them will be now described in greater detail. Each radio block comprises three parts: a MAC header, an RLC block and a Block Check Sequence BCS. The MAC header (octet 1) comprises e.g. an Uplink State Flag USF, a type field T and a Power Control field PC. A value 0 of the type field T denotes an RLC data block and a value 1 an RLC/MAC control block.
The RLC data block (in which T=0) comprises an RLC Header and an RLC information field. The RLC header (octets 2 to 3) denotes e.g. the position of the block in relation to other blocks in the sequence (BSN=Block Sequence Number). In addition, the RLC header comprises allocation and LLC frame data. An Extension bit field E shows whether an optional extension octet 4 is in use. According to the standard proposal, the intended use of the field S/P in connection with data blocks is that when said bit is in the polling state xe2x80x98Pxe2x80x99, the mobile station is to send an acknowledgement (Ack/Nack) of the received blocks. The Temporary Flow Identity field TFI is used in downlink blocks to denote a receiving mobile station. The 7 bits allocated to the TFI field can be used for denoting 127 mobile stations (27=128), because one of the values is reserved for general transmissions received by all mobile stations.
In downlink blocks the USF field is used to provide a mobile station-specific authorization for transmitting a corresponding uplink block. So in a downlink block, the USF field provides an identifier of an uplink counter block. The USF field comprises 3 bits, i.e. it can acquire 8 different values. One USF field value is allocated to the mobile station receiving the packet, to denote the uplink block the mobile station is to use for acknowledging the received radio blocks. At the same time the mobile station can express its wish to use a separate uplink data channel.
A problem with an arrangement described above arises from the USF identifier allocated in connection with the allocation of the downlink resource. Since the USF identifier is also allocated to the receiving mobile station, the same USF identifier value cannot be allocated for uplink transmissions. Since the USF identifier can acquire only a limited number of possible values (i.e. eight different values), each mobile station receiving in downlink prevents the network from allocating a particular uplink mobile station to the channel in question. If at a particular moment one and the same channel is used for reception by 8 separate mobile stations, a corresponding uplink channel cannot have any separate transmissions, even if plenty of air interface capacity would be available.
A possible solution could be to increase the USF field to comprise more than three bits. The problem would thus become less significant, even if the uplink and downlink were not independent of each other. Another alternative solution could be a separate information field (=a bit) indicating whether the USF identifier is in use or not. At least one problem with these solutions would be that the USF field is strongly protected with channel coding, so it uses (with CS-1 coding) up to four times more bandwidth in relation to the net information content. Such an additional load reduces the capacity available to payload traffic.
The invention is firstly based on a detection of a problem, which is a false reasoning behind the standard proposal. In addition to the detection of the problem, an object of the invention is to provide a mechanism that allows the above-mentioned problem to be solved. The object of the invention is achieved with a method characterized by what is stated in the independent claims. The dependent claims relate to the preferred embodiments of the invention.
In the invention, allocation of a Packet Associated Control Channel (PACCH) is not dependent of a possible USF identifier. Instead, a mobile station acknowledges data packets received, after a predetermined time, from a downlink block in which said polling field is in a polling state. When the allocation of the control channel PACCH is independent from the USF identifier allocated to the mobile station, the invention implements a principle of the GPRS system according to which uplink and downlink capacity deployment should be independent of each other. According to the invention, when the mobile station sends the acknowledgement, the USF identifier in the corresponding downlink block must be one that has not been allocated to any mobile station, so as to avoid collision in the transmissions sent by mobile stations.